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Magic Afternoon

Hold somebody’s hand and feel its warmth. Gram per gram, it converts 10 000 times more energy per second than the sun. You find this hard to believe? Here are the numbers: an average human weighs 70 kilograms and consumes about 12 600 kilojoules/day; that makes about 2 millijoules/gram.second, or 2 milliwatts/gram. For the sun it’s miserable 0.2 microjoules/gram.second. Some bacteria, such as the soil bacterium “Azotobacter” convert as much as 10 joules/gram.second, outperforming the sun by a factor 50 million. I am warm because inside each of my body cells there are dozens, hundreds or even thousands of mitochondria that burn the food I eat.